In previous studies, it had been determined that serum neutralization of HIV-1 occurs in human peripheral blood monocytes to the same extent as in human T-cell lines. Using this previously developed system of maintaining long-term cultures of human monocytes, the kinetics of antibody inhibition of in vitro HIV-1 infection of this cell type was studied. Virus was preincubated for various lengths of time with sera from normal or HIV-infected individuals prior to inoculation into cells. In other experiments sera were added to cells after infecting with virus. Productive virus infection was quantitated by measurement of p24 antigen and reverse transcriptase in culture supernatants. These studies indicate that virus neutralization by specific antibodies is virtually complete within two minutes, and that significant neutralization may also take place in cells that are already infected.